Apparatus And Method For Liquefying Natural Gas By Refrigerating Single Mixed Working Medium

ABSTRACT

A system and a method for liquefying natural gas using single mixed refrigerant as refrigeration medium are provided. The system comprises a two-stage mixed refrigerant compressor ( 1 ), coolers ( 21, 22 ), gas-liquid separators ( 31, 32 ), throttling devices ( 51, 52 ), a plate-fin heat exchanger group ( 8 ) and a LNG storage tank ( 9 ). The method of the present invention reduces the power consumption for gas compression by compressing and separating the mixed refrigerant stage by stage. The heat exchange curves of cold fluid and hot fluid in the total heat exchange process match with each other better by the aid of using multiple-stage heat exchange, which can reduce the flow of the mixed refrigerant. Further, the system of the present invention has a good adaptability to load-variable operation of the apparatus, and thus can effectively avoid abnormal liquid-flooding at the bottom of the cold box.

FIELD OF THE INVENTION

The present invention relates to the liquefaction of a hydrocarbon-richgas, and particularly to a method and a system for liquefying naturalgas using single mixed refrigerant as refrigeration medium.

BACKGROUND OF THE INVENTION

Natural gas is becoming the best material to replace other fuels becauseof its environmental friendliness, and its fields of application hasgradually expanded to power generation, automotive gas, industrial gas,domestic gas, chemical gas etc.

With the growth of natural gas consumption, the trade volume ofliquefied natural gas as one of the most effective forms for supplyingnatural gas has become one of the fastest growth areas in the energymarket. Continuous development of liquefied natural gas industry placesstricter requirements for energy consumption, investment, efficiencyetc. of the natural gas liquefaction method and apparatus.

Currently, the relatively well-established techniques for liquefyingnatural gas mainly comprise: cascade refrigeration technology, expansionrefrigeration technology and mixed refrigerant refrigeration process.Among them, single mixed refrigerant refrigeration process is morepreferable for medium-sized LNG plant.

In the existing process for liquefying natural gas by single mixedrefrigerant refrigeration, the refrigerant compressor system comprisestwo compressor stages, and the liquefaction of natural gas is carriedout by using one-stage heat exchange.

In the prior art, as shown in FIG. 1, the system comprises a two-stagemixed refrigerant compressor 1 driven by a motor, two coolers 21 and 22,two gas-liquid separators 31 and 32, two liquid pumps 4 and 4′, aplate-fin heat exchanger 8 and a LNG storage tank 9. The mixedrefrigerant composed of C1˜C5 alkanes and N₂ in a reasonable proportionis fed into the inlet of the compressor, compressed to 0.6˜1 MPa byfirst stage compression, entered into the first stage cooler and cooledto 30˜40° C., and then introduced into the first stage gas-liquidseparator for gas-liquid separation. The gas separated from the top ofthe first stage gas-liquid separator is fed to the inlet of the secondstage compressor, compressed to 1.6˜2.5 MPa by the second stagecompression; and the liquid obtained at the bottom of the first stagegas-liquid separator is pressurized by the first liquid pump, mixed withthe gas from the outlet of the second stage compressor, furtherintroduced to the second stage cooler and cooled to 30˜40° C., and thenthe mixed refrigerant after cooling is fed to the second stagegas-liquid separator for gas-liquid separation. The separated liquid ispressurized by the second liquid pump and mixed with the gas obtained atthe top of the second stage gas-liquid separator. The resulting mixtureis fed to the plate-fin heat exchanger, precooled to a determinedtemperature, throttled after exiting from the heat exchanger, andreturned to the plate-fin heat exchanger again for providing cold energyfor the overall heat exchanging process. The natural gas is supplied tothe LNG storage tank after passing through the plate-fin heat exchanger.

In the above-said process, to ensure that the liquid and the gas is fedto the same passage of the plate-fin heat exchanger and participated inheat exchanging, the liquid obtained at the bottom of the final stagegas-liquid separator needs to be pressurized to overcome the liquidcolumn pressure resulted from the height difference from the liquidoutlet of the bottom of the gas-liquid separator to the refrigerantinlet at the top of the plate-fin heat exchanger, which is achieved byproviding the final stage liquid pump. The heat exchange process betweenthe refrigerant and natural gas in the plate-fin heat exchanger is aone-stage heat exchange, the optimization of the temperature differencefor heat exchange between the streams is limited to some extent, theenergy consumption of the apparatus is high, and abnormalliquid-flooding is easy to occur at the bottom of the cold box. Further,there is no good adaptability to load-variable operation of theapparatus.

SUMMARY OF THE INVENTION

The present invention provides a method and a system for liquefyingnatural gas using single mixed refrigerant as refrigeration medium.Natural gas is liquefied by single mixed refrigerant cooling in thepresent invention.

The method and system for liquefying natural gas using single mixedrefrigerant as refrigeration medium in the present invention have anatural gas cycle and a mixed refrigerant refrigeration cycle. In themixed refrigerant refrigeration cycle, the stage-by-stage compression ofthe mixed refrigerant is accompanied by stage-by-stage gas-liquidseparation, and the liquid phase stream separated from the first stagecompression does not participate in the subsequent compression process,which effectively reduces the power consumption of the subsequent gascompression. The gas phase and liquid phase mixed refrigerant streamsobtained by compression are fed to different passages of the heatexchanger group respectively for throttling and heat exchanging, thefinal stage liquid pump is omitted compared to the existing process, andthe heat exchange curves of cold fluid and hot fluid in the total heatexchange process match with each other better by the aid of usingmultiple-stage heat exchange. The gas phase from the final stagegas-liquid separator is throttled, formed as a backflow, reheated andentered into a refrigerant separator, which may effectively avoidabnormal liquid-flooding of the cold box.

The system for liquefying natural gas using single mixed refrigerant asrefrigeration medium in the present invention comprises a mixedrefrigerant compressor system and a cold box system, wherein the mixedrefrigerant compressor system uses two-stage compression and comprises atwo-stage mixed refrigerant compressor, two coolers, two gas-liquidseparators, and a liquid pump, and the cold box system comprises aplate-fin heat exchanger group (two-stage heat exchange), two gas-liquidseparator (comprising a heavy hydrocarbon separator and a refrigerantseparator) and two throttling devices. The whole heat exchange of themixed refrigerant and natural gas is conducted in the cold box system.

In the mixed refrigerant compressor system, the outlet of the firststage compressor is connected to the first cooler, and the latter isconnected to the first gas-liquid separator. The gas phase port of thefirst gas-liquid separator is connected to the second stage compressor,the liquid phase port at the bottom of the first gas-liquid separator isconnected to a liquid pump, and the output line of the liquid pump isconverged with the output line of the second stage compressor and thenconnected to the second cooler. The second cooler is then connected tothe second gas-liquid separator. The gas phase port at the top of thesecond gas-liquid separator is in fluid communication with the firstheat exchange passage of the heat exchanger group, and the liquid phaseport of the bottom of the second gas-liquid separator is in fluidcommunication with the second heat exchange passage of the heatexchanger group.

In the cold box system, the liquid phase port at the bottom of thesecond gas-liquid separator is connected to an end of the firstthrottling device via the second heat exchange passage of the heatexchanger group, and an another end of the first throttling device isconnected to the first stage compressor via the third heat exchangepassage of the heat exchanger group. The gas phase port at the top ofthe second gas-liquid separator is connected to the first heat exchangepassage of the heat exchanger group for precooling, and then connectedto an end of the second throttling device, and an another end of thesecond throttling device is connected to the refrigerant separator viathe fourth heat exchange passage of the heat exchanger group. A naturalgas line is connected to the fifth heat exchange passage of the heatexchanger group and then to the heavy hydrocarbon separator. The gasphase port at the top of the heavy hydrocarbon separator is connected tosubsequent stages of the heat exchanger and then to the LNG storagetank, and the liquid phase obtained at the bottom of the heavyhydrocarbon separator is obtained as the liquefied petroleum gas (LPG)product.

In the mixed refrigerant compressor system, the gas exited from thefirst compressor stage is passed through the first cooler and cooled,and then entered into the first gas-liquid separator for gas-liquidseparation; the gas phase after separation is passed through the secondcompressor stage, and the liquid phase after separation is pressurizedby a liquid pump, converged with the hot gas obtained after the secondstage compression, cooled by the second cooler, and introduced to thesecond gas-liquid separator for gas-liquid separation; and the gas phaseobtained at the top of the second gas-liquid separator is passed throughthe first heat exchange passage (i.e. the gas phase passage) of thedownstream heat exchanger, and the liquid phase obtained at the bottomof the second gas-liquid separator is passed through the second (liquidphase) heat exchange passage of the downstream heat exchanger. In thecold box system, the liquid refrigerant from the bottom of the secondgas-liquid separator in the mixed refrigerant compressor system ispassed through the heat exchanger group, precooled and then throttled bythe first throttling device; the throttled stream is introduced to themiddle part of the refrigerant separator; the gas refrigerant from thetop of the second gas-liquid separator is passed through the heatexchanger group, precooled and then throttled by the second throttlingdevice; the throttled gas stream is reversely passed through the heatexchanger group, reheated to a determined temperature, introduced intothe middle part of the refrigerant separator, and converged with theprecooled and throttled liquid refrigerant which is exited from thefirst throttling device and entered likewise into the above-mentionedrefrigerant separator. The two refrigerants are separated into twophases, i.e., gas phase and liquid phase, by the refrigerant separator,and the two phases exited from the refrigerant separator are joinedtogether and then returned to the heat exchanger group for providingcold energy. On the other hand, natural gas is firstly passed throughthe heat exchanger group, cooled to a given temperature and then enteredto the heavy hydrocarbon separator for separation. The heavy hydrocarboncomponent is obtained at the bottom of the heavy hydrocarbon separator.The gas phase component obtained at the top of heavy hydrocarbonseparator is further passed through the other stages of the heatexchanger group for heat exchanging, cooled to supercooled state, andthus obtained LNG is delivered to the LNG storage tank for storing.

In order to better understand the present invention, the technicalsolution of the system of the present invention are summarized asfollows.

The present invention provides a system for liquefying natural gas usingsingle mixed refrigerant as refrigeration medium comprising a mixedrefrigerant compressor system and a cold box system,

wherein:the mixed refrigerant compressor system comprises:a two-stage mixed refrigerant compressor;a first cooler and a second cooler respectively connected to the firststage and the second stage of the two-stage mixed refrigerantcompressor;a first gas-liquid separator and a second gas-liquid separatorrespectively connected to the first cooler and the second cooler; anda liquid pump connected to the first stage gas-liquid separator, andthe cold box system comprises:a plate-fin heat exchanger group comprising at least six heat exchangepassages, i.e., the first, second, third, fourth, fifth and sixth heatexchange passages, wherein the inlet ends of the first and second heatexchange passages are respectively connected to the gas phase port andliquid phase port of the second gas-liquid separator via two pipelines,and the outlet end of the third heat exchange passage are connected tothe first stage compressor by pipeline;a first throttling device connected to the outlet end of the second heatexchange passage of the plate-fin heat exchanger group;a second throttling device connected to the outlet end of the first heatexchange passage and the inlet end of the fourth heat exchange passageof the plate-fin heat exchanger group;a refrigerant separator connected to the inlet end of the third heatexchange passage, the outlet end of the fourth heat exchange passage ofthe plate-fin heat exchanger group and the first throttling device;a heavy hydrocarbon separator connected to a separate heat exchangepassage (i.e., the fifth heat exchange passage) of the plate-fin heatexchanger group,the gas phase port of the first gas-liquid separator is connected to thesecond stage of the two-stage mixed refrigerant compressor,the liquid phase discharge line of the first gas-liquid separator isconverged via a liquid pump with the discharge line of the secondcompressor stage, and then connected to the second cooler,the gas phase port and liquid phase port of the second gas-liquidseparator are respectively connected to the inlet ends of two heatexchange passages, i.e., the first and second heat exchange passages ofthe plate-fin heat exchanger group,wherein the first throttling device connected to the outlet end of thesecond heat exchange passage is additionally connected to therefrigerant separator,the gas phase discharge line at the top of the refrigerant separator isconverged with the liquid phase discharge line at the bottom of therefrigerant separator, and then connected to the inlet end of the thirdheat exchange passage, and the outlet end of the third heat exchangepassage is connected to the first stage of the two-stage mixedrefrigerant compressor,the fourth heat exchange passage, which is connected on its inlet end tothe second throttling device, is further connected on its outlet end tothe refrigerant separator,a natural gas line is connected to the heavy hydrocarbon separator viaabove-mentioned separate heat exchange passage, i.e., the fifth heatexchange passage of the plate-fin heat exchanger group, andthe gas phase port at the top of the heavy hydrocarbon separator isconnected to the LNG storage tank after passing through a heat exchangepassage, i.e., the sixth heat exchange passage of the plate-fin heatexchanger.

Optionally, the gas phase port at the top of the heavy hydrocarbonseparator is connected to the LNG storage tank after passing through thesixth and further seventh heat exchange passages of the plate-fin heatexchanger successively. The “first compressor stage” and “first stagecompressor” as described herein can be used interchangeably, and so on.

The method for liquefying natural gas by using the above-describedsystem comprises:

Natural Gas Cycle:

Purified natural gas is firstly passed through the plate-fin heatexchanger group, precooled to −30° C.˜−80° C. and then entered to theheavy hydrocarbon separator for gas-liquid separation,The gas phase stream separated from the top of heavy hydrocarbonseparator is further passed through the other stages of the heatexchanger group for heat exchanging, cooled to −130° C.˜−166° C., andthus obtained LNG is delivered to the LNG storage tank for storing;

Mixed Refrigerant Cycle:

The mixed refrigerant composed of C1˜C5 alkanes and N₂ (usually four orfive or six components selected from C1, C2, C3, C4, C5 alkanes and N₂,and these components are mixed in any volume ratio or in substantiallyequal ratio) is fed into the inlet of the compressor, compressed to0.6˜1.8 MPa by first stage compression, entered into the first coolerand cooled to 30˜40° C., and then introduced into the first gas-liquidseparator for gas-liquid separation;the gas separated from the top of the first stage gas-liquid separatoris fed to the inlet of the second stage compressor, compressed to1.2˜5.4 MPa by the second stage compression;the liquid separated from the liquid phase port at the bottom of thefirst stage gas-liquid separator is pressurized to 1.2˜5.4 MPaA by theliquid pump, mixed with the hot gas from the outlet of the second stagecompressor, further introduced to the second cooler and cooled to 30˜40°C., and then the mixed refrigerant after cooling is fed to the secondgas-liquid separator for gas-liquid separation;the gas obtained at the top of the second stage gas-liquid separator ispassed through the first heat exchange passage of the main heatexchanger group for heat exchanging, and the liquid separated from thebottom of the second stage gas-liquid separator is passed through thesecond heat exchange passage of the main heat exchanger group for heatexchanging;the liquid separated from the bottom of the second gas-liquid separatoris precooled to about −30° C.˜−80° C. in the second heat exchangepassage of the heat exchanger group, throttled to 0.2˜0.8 MPaA by thefirst throttling device, and then introduced to the middle part of therefrigerant separator;the gas phase stream of the mixed refrigerant separated from the top ofthe second gas-liquid separator is passed through the gas phase passageof the heat exchanger group, cooled to −135° C.˜−169° C. and thenthrottled to 0.2˜0.8 MPaA by the second throttling device; the throttledgas stream is reversely passed through the heat exchanger group forproviding cold energy and reheated to −30° C.˜−80° C., and thenintroduced into the middle part of the refrigerant separator afterexiting from the heat exchanger group, and converged, in the refrigerantseparator, with the cooled and throttled liquid refrigerant (exited fromthe first throttling device and entered likewise into theabove-mentioned refrigerant separator). The converged two refrigerantsare separated into two phases, i.e., gas phase and liquid phase, by therefrigerant separator, and the two phases exited from the refrigerantseparator are joined together, returned to the heat exchanger group forproviding cold energy and then introduced into the first compressorstage as the mixed refrigerant.

The pressure unit “MPaA” as described herein refers to Megapaskal,absolute pressure.

The method of the present invention and the system used in the methodhave been described as above sufficiently.

The advantages of the present invention are as follows:

1. The system of the present invention has a good adaptability toload-variable operation of the apparatus, and the gas phase from thefinal stage gas-liquid separator is throttled, formed as a backflow,reheated and entered into a refrigerant separator, which can effectivelyavoid abnormal liquid-flooding at the bottom of the cold box, thusensuring energy consumption at low load conditions is close to energyconsumption at normal operating conditions.2. The method of the present invention uses a two-stage mixedrefrigerant compressor to compress and separate the mixed refrigerantstage by stage, which reduces the power consumption for gas compression.3. The liquid stream obtained at the bottom of the first gas-liquidseparator does not participate in the subsequent stream compressionprocess, which to some extent reduces the influence of the fluctuationof the mixed refrigerant ratio on the compressor running conditions,making the system easier to operate.4. The heat exchange curves of cold fluid and hot fluid in the wholeheat exchange process match with each other better by the aid of usingtwo-stage heat exchange, which can reduce the flow of the mixedrefrigerant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system for liquefying natural gas in the prior art.

FIG. 2 shows the system for liquefying natural gas using mixedrefrigerant of the present invention.

-   1 two-stage mixed refrigerant compressor-   4, 4′ liquid pump-   6 heavy hydrocarbon separator-   7 refrigerant separator-   8 plate-fin heat exchanger group-   9 LNG storage tank-   21, 22 cooler-   31, 32 gas-liquid separator-   51, 52 throttling device

THE MODE OF CARRYING OUT THE INVENTION

The embodiments of the present invention are further described withreference to the Drawings.

Natural Gas Cycle:

As shown in FIG. 2, purified natural gas as raw material is firstlypassed through the fifth heat exchange passage of the plate-fin heatexchanger group 8, cooled to −30° C.˜−80 ° C. and then entered to theheavy hydrocarbon separator 6 for gas-liquid separation.The gas phase stream separated from the top of heavy hydrocarbonseparator 6 is further passed through the other stages of the main heatexchanger group 8 (i.e., the sixth heat exchange passage) for heatexchanging, cooled to −130° C.˜−166° C., and thus obtained liquefiednatural gas (LNG) is delivered to the LNG storage tank 9 for storing.Liquefied petroleum gas (LPG) is obtained at the bottom of the heavyhydrocarbon separator 6.

Mixed Refrigerant Cycle:

The mixed refrigerant composed of C1˜C5 alkanes and N₂ (i.e., four orfive or six components selected from C1, C2, C3, C4, C5 alkanes and N₂,and these components are mixed in any volume ratio or in substantiallyequal ratio) is fed into the inlet of the compressor 1, compressed to0.6˜1.8 MPa by first stage compression, entered into the first cooler 21and cooled to 30˜40° C., and then introduced into the first gas-liquidseparator 31 for gas-liquid separation;the gas separated from the top of the first gas-liquid separator 31 isfed to the inlet of the second stage compressor, compressed to 1.2˜5.4MPa by the second stage compression;the liquid separated from the liquid port at the bottom of the firstgas-liquid separator 31 is pressurized to 1.2˜5.4 MPaA by the liquidpump 4, mixed with the hot gas from the outlet of the second stagecompressor, further introduced to the second cooler 22 and cooled to30˜40° C., and then the mixed refrigerant after cooling is fed to thesecond gas-liquid separator 32 for gas-liquid separation;the gas obtained at the top of the second gas-liquid separator 32 ispassed through the first heat exchange passage of the main heatexchanger group 8 for heat exchanging, and the liquid separated from thebottom of the second gas-liquid separator 32 is passed through thesecond heat exchange passage of the main heat exchanger group 8 for heatexchanging.the liquid separated from the bottom of the second stage gas-liquidseparator 32 is precooled to about −30° C.˜−80° C. in the second heatexchange passage of the heat exchanger group, throttled to 0.2˜0.8 MPaAby the first throttling device 51, and then introduced to therefrigerant separator 7;the gas phase stream of the mixed refrigerant separated from the top ofthe second gas-liquid separator 32 is passed through the gas phasepassage (i.e., the first heat exchange passage) of the heat exchangergroup 8, cooled to −135° C.˜−169° C. and then throttled to 0.2˜0.8 MPaAby the second throttling device 52; the throttled gas stream isreversely passed through the fourth heat exchange passage of the heatexchanger group 8 for providing cold energy and reheated to −30° C.˜−80°C., and then introduced into the middle part of the refrigerantseparator 7 after exiting from the heat exchanger group, and converged,in the refrigerant separator, with the cooled and throttled liquidrefrigerant stream (exited from the first throttling device and enteredlikewise into the above-mentioned refrigerant separator) which isintroduced from the second stage gas-liquid separator 32. The convergedtwo refrigerants are separated into two phases, i.e., gas phase andliquid phase, by the refrigerant separator, and the two phases exitedfrom the refrigerant separator are joined together, returned to thethird heat exchange passage of the heat exchanger group for providingcold energy and then introduced into the first compressor stage as themixed refrigerant.

What is claimed is:
 1. A system for liquefying natural gas using singlemixed refrigerant as refrigeration medium comprising a mixed refrigerantcompressor system and a cold box system, wherein: the mixed refrigerantcompressor system comprises: a two-stage mixed refrigerant compressor; afirst cooler and a second cooler respectively connected to the firststage and the second stage of the two-stage mixed refrigerantcompressor; a first gas-liquid separator and a second gas-liquidseparator respectively connected to the first cooler and the secondcooler; and a liquid pump connected to the first stage gas-liquidseparator, and the cold box system comprises: a plate-fin heat exchangergroup comprising at least six heat exchange passages, i.e., the first,second, third, fourth, fifth and sixth heat exchange passages, whereinthe inlet ends of the first and second heat exchange passages arerespectively connected to the gas phase port and liquid phase port ofthe second gas-liquid separator via two pipelines, and the outlet end ofthe third heat exchange passage are connected to the first stagecompressor by pipeline; a first throttling device connected to theoutlet end of the second heat exchange passage of the plate-fin heatexchanger group; a second throttling device connected to the outlet endof the first heat exchange passage and the inlet end of the fourth heatexchange passage of the plate-fin heat exchanger group; a refrigerantseparator connected to the inlet end of the third heat exchange passage,the outlet end of the fourth heat exchange passage of the plate-fin heatexchanger group and the first throttling device; a heavy hydrocarbonseparator connected to a separate heat exchange passage, i.e., the fifthheat exchange passage of the plate-fin heat exchanger group, the gasphase port of the first gas-liquid separator is connected to the secondstage of the two-stage mixed refrigerant compressor, the liquid phasedischarge line of the first gas-liquid separator is converged via aliquid pump with the discharge line of the second compressor stage, andthen connected to the second cooler, the gas phase port and liquid phaseport of the second gas-liquid separator are respectively connected tothe inlet ends of two heat exchange passages, i.e., the first and secondheat exchange passages of the plate-fin heat exchanger group, whereinthe first throttling device connected to the outlet end of the secondheat exchange passage is additionally connected to the refrigerantseparator, the gas phase discharge line at the top of the refrigerantseparator is converged with the liquid phase discharge line at thebottom of the refrigerant separator, and then connected to the inlet endof the third heat exchange passage, and the outlet end of the third heatexchange passage is connected to the first stage of the two-stage mixedrefrigerant compressor, the fourth heat exchange passage, which isconnected on its inlet end to the second throttling device, is furtherconnected on its outlet end to the refrigerant separator, a natural gasline is connected to the heavy hydrocarbon separator via said separateheat exchange passage, i.e., the fifth heat exchange passage of theplate-fin heat exchanger group, and the gas phase port at the top of theheavy hydrocarbon separator is connected to the LNG storage tank afterpassing through a heat exchange passage, i.e., the sixth heat exchangepassage of the plate-fin heat exchanger.
 2. The system according toclaim 1, characterized in that the gas phase port at the top of theheavy hydrocarbon separator is connected to the LNG storage tank afterpassing through the sixth and further seventh heat exchange passages ofthe plate-fin heat exchanger successively.
 3. A method for liquefyingnatural gas using single mixed refrigerant as refrigeration mediumcomprises: a natural gas cycle wherein: Purified natural gas is firstlypassed through a plate-fin heat exchanger group, precooled to −30°C.˜−80° C. and then entered to a heavy hydrocarbon separator forgas-liquid separation, The gas phase stream separated from the top ofthe heavy hydrocarbon separator is further passed through the otherstages of the heat exchanger group for heat exchanging, cooled to −130°C.˜−166° C., and thus obtained LNG is delivered to a LNG storage tankfor storing; a mixed refrigerant cycle wherein: The mixed refrigerantcomposed of C1˜C5 alkanes and N₂ is fed into the inlet of a two-stagemixed refrigerant compressor, compressed to 0.6˜1.8 MPa by first stagecompression, entered into a first stage cooler and cooled to 30˜40° C.,and then introduced into a first stage gas-liquid separator forgas-liquid separation; the gas separated from the top of the first stagegas-liquid separator is fed to the inlet of the second stage compressor,compressed to 1.2˜5.4 MPa by the second stage compression; the liquidseparated from the liquid phase port at the bottom of the first stagegas-liquid separator is pressurized to 1.2˜5.4 MPaA by a liquid pump,mixed with the hot gas from the outlet of the second stage compressor,further introduced to a second cooler and cooled to 30˜40° C., and thenthe mixed refrigerant after cooling is fed to a second stage gas-liquidseparator for gas-liquid separation; the gas obtained at the top of thesecond stage gas-liquid separator is passed through the first heatexchange passage of the main heat exchanger group for heat exchanging,and the liquid separated from the bottom of the second stage gas-liquidseparator is passed through the second heat exchange passage of the mainheat exchanger group for heat exchanging; the liquid separated from thebottom of the second stage gas-liquid separator is precooled to about−30° C.˜−80° C. in the second heat exchange passage of the heatexchanger group, throttled to 0.2˜0.8 MPaA by a first throttling device,and then introduced to the middle part of a refrigerant separator; thegas phase stream of the mixed refrigerant separated from the top of thesecond gas-liquid separator is passed through the gas phase passage,i.e., the first heat exchange passage of the heat exchanger group,cooled to −135° C.˜−169° C. and then throttled to 0.2˜0.8 MPaA by asecond throttling device; the throttled gas stream is reversely passedthrough the fourth heat exchange passage of the heat exchanger group forproviding cold energy and reheated to −30° C.˜−80° C., and thenintroduced into the middle part of the refrigerant separator afterexiting from the heat exchanger group, and converged, in the refrigerantseparator, with the cooled and throttled liquid refrigerant which isdischarged from the first throttling device and entered likewise intothe above-mentioned refrigerant separator, the converged tworefrigerants are separated into two phases, i.e., gas phase and liquidphase, by the refrigerant separator, and the two phases exited from therefrigerant separator are joined together, returned to the third heatexchange passage of the heat exchanger group for providing cold energyand then introduced into the first compressor stage as the mixedrefrigerant.
 4. The method according to claim 3, characterized in thatthe mixed refrigerant comprises four or five or six components selectedfrom C1, C2, C3, C4, C5 alkanes and N₂, and these components are mixedin any volume ratio or in substantially equal ratio.